1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating thereof, and in more detail a semiconductor laser device having a structure capable of preventing non-emissive failure due to short circuit and a method for fabricating such device.
2. Description of the Related Art
A visible light semiconductor laser device having a stacked structure on a GaAs substrate, wherein an active layer is sandwiched by cladding layers made of AlGaInP or GaInP, has an oscillation wavelength between 630 nm and 690 nm, and attracts a good deal of attention as a light source for an optical pickup used in an optical disc drive.
A structure and fabrication method of a conventional AlGaInP-base visible light semiconductor laser device will be explained hereinafter referring to FIG. 6. FIG. 6 shows a cross-sectional view of the substrate showing a structure of an AlGaInP-base semiconductor laser device.
An AlGaInP-base semiconductor laser device 10 has on a GaAs substrate 12 a stacked structure comprises a lower cladding layer 14 made of n-AlGaInP, an active layer 16, an upper cladding layer 18 made of p-AlGaInP, and a contact layer 20 made of p-GaAs, and all layers are epitaxially grown in this order.
An additional semiconductor layer such as light confining layer may optionally be provided between the upper cladding layer 18 and the contact layer 20. Also a buffer layer made of compound semiconductor may optionally be provided between the GaAs substrate 12 and the lower cladding layer 14.
Of such stacked structure, the upper cladding layer 18 and the contact layer 20 are formed as a mesa-structured portion having a ridge stripe pattern.
The both sides of the upper cladding layer 18 and the contact layer 20 composing the mesa-structured portion, and the upper cladding layer 18 are buried with an n-GaAs layer 22 provided as a current blocking layer to ensure current constriction, thereby a central portion of the active layer becomes an oscillation area 15 of laser light.
A metal layer made of Au, Ni and the like, or a metal stacked film is provided as a p-side electrode 24 on the n-GaAs layer 22 and the contact layer 20, and as an n-side electrode 26 on the rear surface of the GaAs substrate 12, respectively.
In order to fabricate such semiconductor laser device 10, at first the lower cladding layer 14, active layer 16, upper cladding layer 18 and contact layer 20 are epitaxially grown in this order on the GaAs substrate 12 by the metal-organic chemical vapor deposition (MOCVD) process.
The contact layer 20 and the upper cladding layer 18 are then etched to form the mesa-structured portion, and the n-GaAs layer 22 is then selectively grown on the both sides of the mesa-structured portion and on the upper cladding layer 18.
Next, the p-side electrode 24 and n-side electrode 26 are formed by, for example, the sputtering process on the outermost surface and on the rear surface of the GaAs substrate 12.
In the process of epitaxially growing the AlGaInP layer and the like to form the stack-structured portion, there has, however, been a problem of generating a growth defect in the epitaxially grown layer(s) if fine particles of GaAs or so adhere thereon, or foreign intermediate products are formed on the substrate during the epitaxial growth.
In the process of etching the stack-structured portion to form the mesa-structured portion after the epitaxial growth, etching with an acid of such epitaxially grown layer having the growth defect will result in formation of a pit-like shape defect portion 28 of several to tens urn diameter reaching the GaAs substrate 12 as shown in FIG. 7, since the portion of the growth defect is labile to acid and shows a high etchrate etch rate.
If the electrode layer 24 is formed in this situation, the electrode layer 24 intruded into the shape defect portion 28 will come into contact with the GaAs substrate 12 to cause short circuit. Such shape defect portion 28 can be produced in the stack-structured portion made of compound semiconductor layers not only during the wet etching but also during acid cleaning or alkali cleaning based on the same mechanism as described above.
As a result, short circuit will occur between currents injected to the both electrodes, thereby current which essentially has to be injected to the oscillation area in the active layer responsible for laser oscillation is reduced, and it causes non-emissive failures such that no laser oscillation occurs or the laser oscillation does not continue.
It is, however, quite difficult in practice in fabricating the semiconductor laser device to epitaxially grow the compound semiconductor layer after thoroughly cleaning the GaAs substrate and confirming that no particles adhering thereon. Thus so long as the semiconductor laser device is fabricated according to the conventional process, those suffering from non-emissive failures will be more or less produced to degrade the production yield.